Method for promoting dropwise condensation on copper and copper alloy condensing surfaces



United States Patent Inventor David M. Eissenberg Oak Ridge, Tenn. Appl.No. 826,102 Filed June 20, 1969 Patented Dec. 15,1970 Assignee TheUnited States of America as represented by the United States AtomicEnergy Commission.

METHOD FOR PROMOTING DROPWISE I CONDENSATION ON COPPER AND COPPER ALLOYCONDENSING SURFACES 5 Claims, 1 Drawing Fig.

U.S. Cl 165/1, 165/110, 165/133 Int. Cl F28f 13/18 Field of SearchReferences Cited OTHER REFERENCES Drew et al., TB, The Conditions forDrop-Wise Condensation of Steam, Transactions of American Institute ofChemical Engineers Vol. 31. No. 4. 12/25/1935, pp. 613 and 614. TP l.A6(now Chemical Engineering) Hampson, H. Heat Transfer During Condensationof Steam, Engineering, 8/17/1951, p. 221, TA l.E55

Primary Examiner-Albert W. Davis, Jr. AttorneyRoland A. AndersonABSTRACT: A method for promoting dropwise condensation on copper andcopper alloy condensing surfaces is provided. Clean metallic surfacefinishes are developed and preserved on which steam condensate forms indroplets as a result of maintaining a concentration of a few parts permillion of CO at the condensing surface. The CO may be carried by thecondensing vapor or injected directly into the condensing chamber.

AGE OF TUBE, DAYS METHOD FOR PROMOTING DROPWISE CONDENSATION ON COPPERAND COPPER ALLOY CONDENSING SURFACES BACKGROUND OF THE INVENTION Theinvention described herein relates generally to surface condensers andmore particularly to a method of promoting dropwise condensation ofsteam on copper and copper alloy surfaces. It was made in the course of,or under, a contract with the U. S. Atomic Energy Commission.

When a condensing surface is coated with a substance which prevents thecondensate from wetting the surface, the vapor will dense condense indrops, or dropwise, on the condensing surface rather than in the form ofa continuous film. Such dropwise condensation, as opposed to thecontinuous film or filmwise condensation, has, long been considereddesirable because of the relatively large heat transfer coefficientsassociated therewith. Heat transfer coefficients achieved under dropwisecondensation conditions have been found to be from four to eight timesas high as with filmwise condensation.

it has been found that condenser tubes plated with noble metals such assilver, gold and platinum promote dropwise condensation under long termconditions with high purity water. Such materials are prohibitivelyexpensive, however, and as a result have not been consideredcommercially practical as plating materials for promoting dropwisecondensation. Certain organic materials such as waxes and silicones havealso been observed to promote dropwise condensation but are alsoconsidered impractical in thatthey do not form a permanent coating andcannot be economically replenished. Plastic coatings such aspolytetrafluorethylene and polyparaxylylene are known to promotedropwise condensation, but have proven undesirable for that purpose inthat at practical coating thicknesses, they act as insulators andthereby offset any gains achieved as a result of the-dropwisecondensation.

It is, accordingly, a general object of the invention to provide a meansfor promoting dropwise condensation on a condensing surface formed of orplated with an inexpensive commercially available metal.

Other objects of the invention will be apparent from-an examination ofthe following description of the invention and the appended drawing. 1

SUMMARY OF THE INVENTION In accordance with the invention, a method forpromoting dropwise condensation on copper. and copper alloy condensingsurfaces is provided. Clean metallic surfaces accompanied by dropwisecondensation are developed and conserved by maintaining a concentrationof'a few parts per million of CO at the condensing surface. Dropwisecondensation is thus achieved without the need for forming or coatingthe condensing surfaces with expensive metals having inherent nonwettingcharacteristics or providing them with semipermanent organic surfacecoatings.

BRIEF DESCRIPTION OF THE'DRAWING The FIG. is a graph of heat transfercoefficients as a function of time for a condensing system-operated inaccordance with theinvention.

DETAILED DESCRIPTION Although the exact mechanism of the invention isnot known with certainty by applicant, it is believed that the COprovided in accordance with the invention dissolves in the condensate toform a weak carbonic acid solution. The carbonic acid dissolves copperoxide as it forms on the condensing surface, thereby maintaining thesurface clean and suitable for dropwise condensation. Although otherexplanations may be found to account for the effect discovered byapplicant, such explanations will in no way affect the scope ofapplicants invention which is limited only by the appended claims. Inpractice, others may use applicants invention by providing COconcentrations as taught herein without regard to its exact operatingmechanism.

The invention will be more specifically illustrated by means of thefollowing examples:

EXAMPLE I A sample of l-inch outside diameter commercial copper tubingwas installed in a test condenser. A small patch, about 1 inch square,was cleaned with emery paper to remove surface oxide. Steam at 215 and15.5 p.s.i.a. from the plant steam system was admitted to the condenser.Within 24 hours, the cleaned area was condensing in dropwise fashionwhile the remainder of the tube condensed filmwise. After 3 days, theentire tube was condensing dropwise.'The system was shut down and airadmitted. Upon restarting the system, the entire tube surface wasobserved to revert to filmwise condensation. The tube subsequentlyreturned to dropwise condensation within a few hours after restarting.Noncondensables in the steam were analyzed as percentCO-i, 18 percent N2and less than 2 percent 0 with a trace of H The total concentration ofnoncondensables in the steam was about 5 parts per million.

EXAMPLE u A -10 copper-nickel condenser was treated in the mannerdescribed in example I. This tube exhibited the same conversion fromfilmwise to dropwise condensation as the commercial copper tubing ofexample 1.

EXAMPLE III In another test, a 304 stainless steel condenser tube wastested in the manner described in example I. The stainless steel tubefailed to convert from fihnwise to dropwise condensation and the testwas terminated after one week. EXAMPLE ln another series of tests, aspecial steam generator was used to provide better control over the kindand concentration of noncondensables present in the steam. The steamgenerator used deaerated, demineralized water for conversion to steam. A90-10 copper-nickel tube specimen was placed in the condenser and steamfrom the generator admitted. Condensation on the tube specimenwasobserved to be filmwise for several days. The tube was observed tohave a dark copper oxide color during this portion of EXAMPLE test.Carbon dioxide was then admitted to the steam feed stream in an amountestimated to be a few parts per million. After a few hours the darkcopper oxide color began to turn to a patchy pink color and condensationwas observed to convert from the filmwise to the dropwise mode. Duringsubsequent shutdown periods, the condenser tube was observed to darkenbut upon each startup the color improved and filmwise condensationconverted to dropwise. A later addition of hydrogen to the COZ-steammixture had no observable effect on the results achieved with CO above.

EXAMPLE V In another run, hydrazine was bled into the steam along withC0 The final result was the same as in example IV except that the oxidecoating appeared to clear somewhat faster. lt is thus postulated thathydrazine or other reducing agents may be desirable additives along withCO during the initial startup l copper-nickel tube was tested for anextended period in a I vertical tube evaporator test assembly. lPlantsteam, having the same noncondensable content as described in example I,was fused as' the steam supply. Heat transfer coefficients were jd'etermined immediately after startup and again when the oxide film onthe tube began to lighten in color and dropwise fcondensationwas firstobserved. Further heat transfer coeffi- "cient determinations were madeover a 6-day period until condensatio'ri'appe a'red to be entirelydropwise. The heat transfer j c oefficients obtained during the test runare plotted in the I F IG. The heat transfer coefficients plotted in theFIG. are nominal, overall, average coefficients based on nominal out- Inanother test, a 3-inch outside diameter doubly fluted 90- side tubecircumferences and nominal temperature dif- "ferences between thecondensing steam and coolant liquid (discharged from the tube asinferred from pressure measureents, and they include tube-wallresistances. The slight reduction in overall heat transfer coefiicientswhich occurred .afterthe seventh dayresulted when a 4.9 weight percentNaCl solution was substituted for the evaporator feedwater. ,Althoughthe improvement in heat transfe-ris good, it is noted .that thisparticular tube design is superior to conventional tube design andtherefore the improvement over conventional tubes is even greater. Goodperformance of the tube was sustained under test conditions over a 6-dayperiod whereas in the prior art, the performance of all condenser tubeshas been .observed to deteriorate after a short period due to surfaceoxide buildup.

Although the invention is especially desirable for use in:Iargecondensers such as are used and/or planned for use ingdesalination plants of the evaporator type, it 'may be used in .anyheat transfer device in which heat is removed from a -vapor through awall, causing the vapor to condense on the wall. Various means may beused for maintaining the desired concentration of CO at the condensingsurfaces including the injection of CO directly into the condenserchamber or letting it be carried by the condensing vapor. It is alsonoted that the CO concentration in a condenser should be kept as low aspossible consistent with the condenser tubes operating in a dropwisecondensation mode. Concentrations exceeding that minimum valuesufficient to bring'about and maintain dropwise condensation areundesirable since the presence of any noncondensible in the condensertends to lessen its effciency. Thus, although a value of about 5 partsper million has been found to be workable by applicant under testconditions, lesser concentrations may be adequate during steady stateoperation or in other systems where very little oxygen is present in thecondenser.

Iclaim:

1. In a heat transfer system wherein water vapor is condensed bycontacting it with a copper base cooling surface, the improved methodforpromoting dropwise condensation of said vapor on said cooling surfacecomprising maintaining a sufficient concentration of CO in the watervapor adjacent to said cooling surface to cause dropwise condensation tooccur.

2. The improved method of claim 1 wherein said concentration of CO inthe water vapor adjacent to said cooling surface

